The present invention relates to automotive spaceframes and, more particularly, to a method and apparatus for constructing spaceframes by Electromagnetic Pulse Forming (EMF) or Magnetic Pulse Welding (MPW).
The most efficient method, from a cost and weight standpoint, of fabricating an automotive structure is to first construct an inner skeletal structure called a spaceframe. The outer body panels are then attached to the spaceframe, via welding, structural bonding, or other methods. The outer body panels, such as the front and rear fenders, roof, etc., most often have been attached by conventional welding techniques.
Most, if not all, aluminum spaceframes have been constructed using these conventional welding techniques, as can be observed in the fabricating process used to construct the Audi A8 automobile.
An economical method of constructing such a spaceframe is to fixture cast aluminum fittings, often referred to as xe2x80x9cnodesxe2x80x9d. To these fixtured nodes, closed section members are assembled. The closed section members can be extruded, drawn, or seam welded.
One drawback of using conventional welding for spaceframes, particularly those made of aluminum, is the inherent distortion, caused by the heat generated in the welding process. This is true even when fixtures are used at key positions of the spaceframe to brace the parts against the distortion. The fixturing is only partially successful, because once the spaceframe is removed from the fixture, it will nevertheless become distorted from induced residual stresses. The distorted frame will then require straightening, which is both expensive and labor intensive.
Another drawback of the welding process is the weakening of the spaceframe by the generated heat required for the welding, which anneals the aluminum material in the xe2x80x9cheat affected zonexe2x80x9d. Unfortunately, these heat affected zones are those areas that are often subjected to the highest mechanical stresses.
In order to correct for this, additional aluminum material is often added to strengthen the structure in the welded joint area. This is not a good solution to the problem, however, because the additional materials increase the cost and weight of the finished vehicle.
Still another drawback of the spaceframe design that employs conventional welding techniques is that aluminum cannot be welded to steel. One solution for attaching aluminum to steel is the use of electromagnetic force. The force is applied to a steel band that is fitted over the dissimilar components of a frame coupling, as is illustrated in U.S. Pat. No. 5,966,813, issued to Durand, on Oct. 19, 1999, for a METHOD FOR JOINING VEHICLE FRAME COMPONENTS. The band is caused to distort under the applied electromagnetic forces, and bond the frame components together.
The use of Electromagnetic Pulse Forming (EMF) or Magnetic Pulse Welding (MPW) eliminates the aforementioned drawbacks of conventionally welded spaceframe construction. There is no heat inherent in either process, so that the need to fixture and straighten the joined components after assembly is eliminated.
There are several drawbacks in the above mentioned Durand process, one of which is the use of a steel band. The band adds to the cost and complexity of the welding process, and does not provide a strong bond between the parts themselves. The use of a thin band does not provide sufficient mechanical strength for joining areas subject to high stress.
In a second instance, placing the band at the areas to be joined is cumbersome, and does not lend itself to fabricating the spaceframe along mass-production assembly lines. The electromagnetic coils providing the electromagnetic force for forming the band cannot be removed from, or negotiated about many of the joined areas along the frame. Thus, the Durand process would not easily conform to current automotive assembly line, mass production techniques.
The use of only dissimilar materials in the Durand process is also a drawback, since automobiles like the Audi A8 use an entire aluminum spaceframe.
A process described in U.S. Pat. No. 3,520,049 to Lysenko et al, which is referred to as Magnetic Pulse Welding (MPW) is similar to EMF, wherein magnetic fields induced by high electrical currents are used to join the materials. In the case of MPW, higher forces are generated and the materials are molecularly welded together. A torque-carrying MPW joint is shorter and therefore lighter than an equivalent performance EMF joint.
In the MPW process, precise positioning of the components is required to produce a consistent weld. A loose fit between the components, as described in U.S. Pat. No. 5,981,921, requires accurate tooling to support the individual components to be welded. The described invention precisely aligns and positions the parts relative to each other without external holding fixtures, thus ensuring a high quality weld.
The present invention seeks to overcome the many drawbacks of the above processes. The current invention uses a split coil. This allows the apparatus to be positioned and removed along the automotive assembly line. Owing to the fact that a spaceframe is a closed structure, a split coil (EMF) or split inductor (MPW) is required to install and remove the forming device before and after the assembly process.
The invention does not require a band to join the frame members. Rather, the inventive process uses undercuts and raised lands along the surface of the parts, in order to more securely join them together. In addition, the present process can be used for both similar and dissimilar materials. Furthermore, the present process fabricates a much stronger weld, wherein, in the joined area, the parts actually fuse together at the molecular level.
The invention allows for permanently joining aluminum to steel, provided that considerations are made to properly protect the interface between the aluminum and steel from corrosion due to galvanic action, as may occur to dissimilar metals. These concerns are easily addressed using conventional painting or sealing techniques in the joint areas.
Further, the process of this invention is capable of assembling aluminum members to nodes made of injection molded high performance composite plastic material which further reduces cost and weight, while eliminating concerns of galvanic corrosion.
In accordance with the present invention, there is provided a method of manufacturing an automotive inner-skeletal structure, commonly referred to as a xe2x80x9cSpaceframexe2x80x9d, using Electromagnetic Pulse Forming (EMF) or Magnetic Pulse Welding (MPW) techniques. The spaceframe can comprise either an all aluminum structure, or a combination of dissimilar materials, such as aluminum and steel. The spaceframe can comprise cast aluminum nodes that are fixtured. The nodes are designed to receive aluminum extruded, drawn, or seam welded closed members. The closed members are assembled to the protruding branches of the nodes. The nodes can also comprise high strength steel, stainless steel, titanium, or magnesium. The closed members can comprise aluminum and can surround protruding branches projecting from the nodes. The nodes are permanently joined to the branches using either an EMF or MPW process.
The nodes can additionally be made of plastic (non-metallic) material with connection branches, and the aluminum closed members can surround the connection branches. The closed member can be permanently joined to the connection branches using EMF. In at least a portion of the spaceframe, the branches of an aluminum node of the spaceframe can surround a steel connection member. The branch can be permanently attached to the steel member using either EMF or MPW. This would allow for the incorporation of one, or several, high strength steel members into a spaceframe that would otherwise consist of aluminum components.
The split coils and split inductors used to fabricate the spaceframe make mass production and assembly line techniques more feasible when using EMF and MPW. The split coils can be stationary, and the joints presented to them for assembly via a moving fixture; or the split coil or inductor can be moved to each joint of a stationary, pre-assembled spaceframe. NBG Technologies/TechnaPulse has successfully manufactured and tested split coils and split inductors and they are not the subject of this patent.
The nodes of the invention are fabricated with a combination of undercuts, or grooves, and raised surface lands. During pulse welding, the opposing structured surfaces bond together at a molecular level creating an exceptionally strong bond.
It is an object of the present invention to provide an improved automotive spaceframe fabrication method.
It is another object of this invention to provide a fabrication process for an automotive spaceframe that permanently attaches the nodes to the closed members by Magnetic Pulse Welding or Electromagnetic Pulse Forming.
It is a further object of the invention to provide a fabrication process for an automotive spaceframe that results in a stronger and lighter structure.